1. Field of the Invention
The present invention relates to a resistive nonvolatile memory element in which nanoparticles are used, and a method for producing the element in which a protein-inorganic nanoparticle complex is used.
2. Related Art
Although resistive memory elements are expected as one of novel nonvolatile memory elements, problems of needs for forming process (process of applying a high voltage after producing the element to initialize the characteristics of the element), and a large variation of characteristics among the elements are often caused. The forming process is considered as a process for forming a local electric current path inside the element, and the differences of the states of the produced electric current paths are believed to be responsible for such variation of characteristics. In addition, reduction of the electric current value required for the memory operation would be preferable since electric power consumption in connection with the memory driving can be reduced. When the electric current density of the element can be reduced, the electric current value to be required can be decreased even with elements having an identical size. The term “electric current density of element” herein means the electric current density of the element, as a whole, i.e., a value derived by dividing the electric current value required for the element operation by the area of the entire element, unless otherwise described in particular.
In order to solve the problems of needs for the forming process and variation of characteristics, techniques of constricting an electric current by forming a local electric current path inside the element beforehand, or arranging fine structures that will lead to triggering of the electric current path formation have been already reported so far. Since constricting of the electric current path reduces the electric current density of the element, it also results in an effect of reducing the electric power consumption.
Patent Document 1 discloses a structure provided with a recessed part or a protruding part on the interface between a resistive layer and an electrode. In addition, a structure including metal fine particles arranged on the top of the protruding part (FIG. 2) has been disclosed. According to Patent Document 1, the voltage of the electrical pulse required for resistance alteration can be reduced, and variation of the electrical pulse width can be suppressed.
Patent Document 2 discloses a structure provided with a resistive layer between a first electrode having a nanochip (ultrafine projection) and a second electrode. Furthermore, a structure provided with a material of different type between a nanochip and a resistive layer (FIG. 3) has been disclosed. According to Patent Document 2, bipolar switching performance can be improved, whereby weak electrical pulse can be used with a low voltage.
Patent Document 3 discloses a structure having a resistive layer between two electrodes, with one electrode being provided with a protruding electrode material. According to Patent Document 3, electric power consumption in writing/erasing can be reduced, and thus a memory element can be formed with favorable reproducibility which achieves stable switching operation without occurrence of failure in writing due to low resistance.
In Nonpatent Document 1, a structure including on a bottom electrode, an insulating layer having micropores in part (mazelike nanogap insulator), and a resistive layer and an upper electrode arranged thereon has been disclosed. According to Nonpatent Document 1, high speed writing/erasing is enabled without need of the forming process.
Unlike control of the electric current path described above, a technique of increasing the rate of resistance change and also suppressing the variation thereof by increasing the region of the crystal interface of the resistance-altering material, thereby making the crystal size uniform has also been already reported.
Patent Document 4 discloses a structure in which a resistive layer and a laminating film having an electric resistivity different from that of the resistive layer are alternately laminated. Furthermore, a structure in which crystals of the resistive layer are arranged which were grown from the metal particles on the electrode as crystal cores (FIG. 4) has been disclosed. Also, a structure in which metal particles (island growth cores) are arranged via a surface tension-adjusting film having a tunnel effect on an electrode (FIG. 5) has been disclosed. According to Patent Document 4, the ratio of electric resistivity in the high resistance state and the low resistance state (CER value) can be increased by providing a large number of junction interfaces, and also variation of the CER value can be depressed by making the crystal size uniform.
In addition, a technique of use of a resistance-altering material in combination with arrangement of fine particles utilizing a protein has also been reported.
Patent Document 5 discloses a structure formed by arranging fine particles on a columnar electric conductor embedded in a dielectric material utilizing ferritin. According to Patent Document 5, reliability with respect to insulation between top and bottom electrodes can be enhanced. In Patent Documents 8 and 9, nonvolatile resistive memory elements having a layer structure of: bottom electrode/resistive layer/tunnel barrier layer/upper electrode have been disclosed.
Patent Document 1: pamphlet of International Publication No. 2005/041303 (particularly, FIG. 6)
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2006-203178
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2007-180473
Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 2007-180174 (particularly, paragraph number 0065)
Patent Document 5: Japanese Unexamined Patent Application, First Publication No. 2006-210639
Patent Document 6: Japanese Unexamined Patent Application, First Publication No. 2006-196601
Patent Document 7: Japanese Unexamined Patent Application, First Publication No. 2004-111734
Patent Document 8: Japanese Unexamined Patent Application, First Publication No. 2007-288008
Patent Document 9: Japanese Unexamined Patent Application, First Publication No. 2008-021750
Nonpatent Document 1: Ogimoto et al., Appl. Phys. Lett. 90 (2007) 143515